Different retroviruses have been isolated from various primate species by cocultivation of tissues with heterologous cells. These isolates include type C viruses from several species of baboons (Papio) and from the closely related genus, Theropithecus. In addition, a group D retrovirus has been isolated from a langur monkey (Presbytis) cell line. This isolate is related to Mason-Pfizer virus (MPMV), but can be readily distinguished from MPMV by its host range, antigenic properties, and by nucleic acid hybridization. Various isolates from squirrel monkey (Saimiri) cell lines have also been obtained. These latter viruses are endogenous to squirrel monkeys, and are present in multiple copies in the cellular DNA of all squirrel monkey cells and tissues. Three new retroviruses have been isolated from lung and spleen cells of the Southeast Asian mouse, Mus cervicolor. All three viruses are endogenous to this rodent species, and related nucleic acid sequences can be detected in the DNA of other rodent species. Two of the isolates, designed CERV C I and CERV C II, are typical type C viruses, while the third isolate shares certain morphologic and biochemical properties with murine type B viruses. BIBLIOGRAPHIC REFERENCES: Todaro, G.J., Sherr, C.J. and Benveniste, R.E.: Baboons and their close relatives are unusual among primates in their ability to release nondefective endogenous type C viruses. Virology 72: 278-282, 1976. Benveniste, R.E. and Todaro, G.J.: Evolution of type C viral Genes: Evidence for an Asian origin of man. Nature 261: 101-108, 1976.